1. Field of the Invention
This invention relates to fuel ignition systems, and more particularly to a control arrangement for use in such systems for controlling the operation of fuel valves of the system.
2. Description of the Prior Art
Various safety arrangements have been proposed for use in fuel ignition systems to prevent inadvertent energization of fuel valves of the system under fault conditions, such as a component failure in an electronic flame sensing circuit, or a welded contact failure of a flame relay. One such control arrangement for a pilot ignition type control system is disclosed in my U.S. Patent application Ser. No. 698,162, now U.S. Pat. No. 4,077,762. In this system, a pilot valve is energized over a path including normally closed contacts of a flame relay in response to closing of thermostatically controlled contacts. When a pilot flame is established, the flame relay is operated under the control of a flame sensing circuit to energize a main valve. A control relay, which is also energized over the path, including the normally closed contacts of the flame relay, provides a holding path for the pilot valve when the flame relay operates. In another control arrangement which is disclosed in my U.S. Pat. No. 4,035,134, a pilot valve is energized via contacts of a control relay which in turn is energized over a path including normally closed contacts of a flame relay.
A common feature of these arrangements is that the control relay is energized over a path which includes normally closed contacts of the flame relay. Thus, the control relay can be energized only when the flame relay is deenergized, and once energized, contacts of the control relay provide a holding path which permits the control relay to be maintained energized until the end of the heating cycle. If for any reason the flame relay is operated in the absence of a flame, or the normally closed contacts are open at the start of an ignition cycle, the energizing path for the control relay is interrupted so that the pilot valve cannot operate on the next call for heat. Also, through the use of a redundant pilot and main valve arrangement, 100% shut off of fuel is effected when the pilot valve is maintained deenergized.